Dehumidification of moist gas mixtures is necessary in a multitude of technical fields. For instance, ventilation and air conditioning of buildings or vehicles generally necessitates not only cooling but also dehumidification of the air since the air to be cooled is often so humid that during cooling to the desired temperature the temperature falls below the dew point temperature. Hence in conventional air conditioning systems dehumidification of the air accounts for a large part of the electricity consumption.
The electricity consumption of air conditioning systems for buildings is reduced by dehumidifying the air by adsorption or absorption of water with a drying medium and subsequently regenerating the water-laden drying medium by heating to a temperature at which the water is desorbed again. Compared to adsorption on a solid adsorbent, the advantage of absorption in a liquid absorption medium is that air dehumidification can be carried out with reduced equipment complexity and with less drying medium and that regeneration of the water-laden drying medium is easier to carry out using solar heat.
A further technical field where dehumidification of moist gas mixtures is employed is the field of absorption chillers (principle described in WO 2014/079675 A1; “absorption chiller” is used synonymously with “absorption heat pump” according to the invention). Here, the moist gas mixture is formed during evaporation of water under low pressure. The water vapour thus formed needs to be removed from the moist gas mixture so that said mixture may then be returned to the water evaporation to pass through a new cycle. Here too, absorption in a liquid absorption medium is favoured over adsorption on a solid adsorption medium.
In addition to use in absorption chillers, there is still a further area in which the drying of gas streams is necessary. This is natural gas refinement, which applies other requirements on the ionic liquids to be used therefor.
To absorb moisture with the aid of ionic liquids, the prior art proposes a series of ionic liquids. Thus, DE 10 2010 004 779 A1 mentions 1-ethyl-3-methylimidazolium ethylsulphate, 1-ethyl-3-methylimidazolium methylsulphate, 1-ethyl-3-methylimidazolium diethylphosphate, 1-ethyl-3-methylimidazolium methylsulphonate, 1-butyl-3-methylimidazolium bistrifluoromethanesulphonimide, 1-butyl-3-ethylimidazolium chloride for example.
CN 102335545 A describes a series of ionic liquids based on alkyl phosphates, namely inter alia 1,3-dimethyimidazolium dimethylphosphate, 1-ethyl-3-methylimidazolium dimethylphosphate and 1-butyl-3-methylimidazolium dimethylphosphate.
Y. Luo et al., Appl. Thermal Eng. 31 (2011) 2772-2777 proposes using the ionic liquid 1-ethyl-3-methylimidazolium tetrafluoroborate in place of aqueous solutions of lithium bromide for air dehumidification. However, this ionic liquid has the disadvantage of only poor absorption capability.
Y. Luo et al., Solar Energy 86 (2012) 2718-2724 proposes the ionic liquid 1,3-dimethyimidazolium acetate as an alternative to 1-ethyl-3-methylimidazolium tetrafluoroborate for air dehumidification. The acetates are also discussed by M. Kanakubo et al., J. Mol. Liq. 217 (2016) 112-119.
US 2011/0247494 A1 proposes a further ionic liquid for this purpose, namely 1-ethyl-3-methylimidazolium acetate.
The application of these ionic liquids in absorption chillers is also discussed in WO 2013/050242 A1.
While these ionic liquids described in the prior art are very well-suited for dehumidification, the ionic liquids, which include dialkylimidazolium ions, exhibit the disadvantage that, especially on contact with gas mixtures, chemicals are given off which lead to olfactory contamination of the ambient air. This is especially the case on dehumidification of indoor air and is undesirable in the case of absorption chillers such as refrigerators. Therefore, an absorbent is sought with which this contamination can be minimized.
For this reason, the object of the present invention consisted of providing an absorbent in which the tendency to give off badly odorous substances therefrom is particularly low. An absorption medium which achieves this object has now surprisingly been found.